In Alzheimer disease (AD), the appearance of neurofibrillary tangles (NFTs) — comprising aggregates of hyperphosphorylated tau — parallels the development of neuronal loss and cognitive decline, leading to speculation that NFTs might be pathogenic. However, it has also been suggested that tau aggregation might be part of a homeostatic response to neuronal damage. In addition, neuronal subtypes and biochemical pathways seem more susceptible to NFTs in some brain areas than others — but the mechanisms of this are poorly understood. Here, Otero-Garcia et al. developed an approach that combined high-throughput fluorescence-activated cell sorting (FACS) with single-cell RNA sequencing (scRNA-seq) to compare the molecular signatures of NFT-containing and NFT-lacking soma in tissue samples from individuals with and individuals without AD. They found that susceptibility to cell death was broadly similar between NFT-containing and NFT-free neurons, but marked associations were found between the presence of NFTs and indicators of cellular stress and synaptic dysfunction.
Having generated an enriched soma fraction prepared from Brodmann area 9 (BA9) neocortical tissue samples from eight individuals with Braak stage VI AD, the authors used the cytoplasmic neuronal marker MAP2 and the AT8 antibody (which detects hyperphosphorylated tau) to separate NFT-bearing (AT8+) and NFT-free (AT8−) somas using a high-throughput FACS-based method. Molecular profiling of ~25,000 AT8+ and ~38,000 AT8− somas followed by cluster analysis showed that markers for all major neuronal subtypes — inhibitory and excitatory — were present in both AT8+ and AT8− somas. Looking at the relative proportion of AT8+ and AT8− somas in each cluster and at tissue histology of sections from the same tissue blocks used for the transcriptome analysis, the authors determined that the excitatory subtypes with the greatest susceptibility to tau pathology included CUX2 in cortical layers 2–4 and RORB/PCP4 in layer 5. Inhibitory neurons were mostly NFT-free, except for ~6% of chandelier cells.
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